Nanoscale optical reflectivity of CVD grown graphene. Sample courtesy of Oak Ridge National Laboratory.
2D materials have become a major emerging field of research thanks to their exceptional properties for key applications in battery technology, semiconductors, photovoltaics and many other areas.
Various microscopy and nanoscale techniques have been used to characterize these 2D materials in order to gain a better insight into the nature of their properties. nanoIR methods extend this characterization with critical nanoscale optical and chemical property mapping.
The nanoIR2-s system provides two complimentary nanoscale IR techniques, scattering-scanning near field optical microscopy (s-SNOM) and AFM-IR photothermal based nanoscale IR spectroscopu imaging and spectroscopy, including Tapping AFM-IR. These methods provide a better understanding of the nanoscale chemical and complex optical properties of 2D materials.
Thermal and mechanical property mapping is an example of complimentary atomic force microscopy technique (AFM) which also provides information on the mechanical, thermal and electrical properties of 2D materials. These methods allow 10 nm spatial resolution optical and chemical property mapping, which is much below the diffraction limit of traditional IR spectroscopy.
This article shows how the nanoIR2-s system is applied to characterize a wide range of 2D materials such as graphene, nanoantennae, semiconductors and more.
Complimentary nanoscale IR Techniques
The nanoIR2-s system is designed to acquire IR spectra and nanoscale images using two separate nearfield spectroscopy techniques –s-SNOM and Photothermal AFM-IR. These are complementary techniques that provide nanoscale chemical analysis, as well as thermal, optical, mechanical and electrical mapping with spatial resolutions down to a few nanometers for both hard and soft matter applications.
Nanoscale IR spectroscopy combines the nanoscale capabilities of AFM with the precise chemical identification of infrared spectroscopy to chemically identify sample components with a chemical spatial resolution down to 10 nm with monolayer sensitivity breaking the diffraction limit by >100x.
Additionally, AFM-IR absorption spectra are direct measurements of sample absorption, independent of other complex optical properties of the tip and sample. As such, the spectra correlate very well to that of traditional bulk transmission IR.
Figure 1. (a) AFM height image shows homogeneous hBN surface with different layers on Si substrate; (b) s-SNOM amplitude shows strong interference fringes due to propagating SPhP along the surface on hBN; (c) s-SNOM phase shows a different phase signal with layer thickness.
Imaging of Plasmons and Phonons
Surface plasmon polaritons (SPPs) and surface phonon polaritons (SPhPs) in 2D materials, with their high spatial confinement, can open up new opportunities for enhanced light-matter interaction, super lenses, subwavelength metamaterial and other novel photonic devices.
A versatile optical imaging and spectroscopy tool with nanometer spatial resolution is required for in situ characterization of these polaritonic excitations across various applications. Through a non-invasive near-field light-matter interaction, the s-SNOM technique offers a unique way to selectively excite and locally detect vibrational and electronic resonances in real space.
This method is shown by imaging the SPhPs of hexagonal boron nitride (hBN), as depicted in Figure 1. Amplitude and phase near-field optical images offer complementary data for detailed characterization of the polaritonic resonances. hBN shows >90° phase shift of SPhPs, suggesting a strong light-matter coupling.
Just like the visualization of the SPhPs in hBN, the graphene SPPs can also be analyzed using the nanoIR2-s system. Shown in Figure 2 is the standing wave of an SPP on a graphene wedge. Usually, the spatial resolution of s-SNOM is restricted only by the end radius of the AFM probe, allowing the s-SNOM method to measure the cross sections of the SPP down to ~8 nm.
Figure 2. (a) s-SNOM phase image of surface plasmon polariton on graphene. (b) Cross section of SPP standing wave phase.
Nanocontamination of Graphene
The exceptional mechanical and electrical properties of graphene are dependent on maintaining the overall conjugated structure of the sheet. The nanoIR2-s system can easily evaluate the quality of exfoliated graphene obtained by various methods as shown in Figure 3.
Figure 3. (a) AFM height image of exfoliated graphene and (b) s-SNOM reflection image, showing nanocontamination (dirt).
Contamination that cannot be easily detected in the AFM height image is observed in the s-SNOM reflection image; furthermore, contrast in the s-SNOM reflection image varies with the number of graphene layers present, showing nanocontamination on the sample.
Characterizing Nanoantenna Resonance
The applications of nanoantennae are very diverse, ranging from sensing to energy conversion. When it comes to the construction of reliable, accurate devices, the ability to measure and adjust the resonance structures of these antennas is very important.
Arrays of nanoantennae are relatively common because they allow for the packing of a large number of individual antennas in a compact area. Shown in Figure 4a is an AFM topography image of an antenna array comprising of both single bar antennas and coupled antennas.
Figure 4. (a) AFM height image of assembled antenna array. (b) s-SNOM amplitude and (c) s-SNOM amplitude images of antenna dipole.
Figure 5. AFM-IR spectrum collected on single rod and coupled antenna; the peak at 910 cm-1 corresponds to the antenna resonance of the single rod antenna, while the peak at 1100 cm-1 shows the Si-O mode shared by both antennas.
When fabricating antenna arrays, optimum energy transfer efficiency can be achieved by considering the contact point to the antennas. s-SNOM imaging makes it easy to detect the antenna resonance hot spots – the ideal contact point. The s-SNOM amplitude and phase image of a single bar antenna contained within the array is shown in Figure 4b. 11 µm excitation is observed at the dipole antenna resonance; the ~180° phase change is also observed at dipole resonance.
Besides the ability to obtain high resolution images of optical phenomenon, the nanoIR2-s system also makes it possible to spectrally probe the features of nanoscale surface. Shown in Figure 5 is the AFM-IR spectra collected on the coupled antenna and single rod, and the antenna resonance can be distinctly resolved at 910 cm-1, in agreement with theoretical predictions.
Effects of Polarized Light on Metasurface Chirality
This is the first time where the combination of the complimentary nanoscale imaging techniques – s-SNOM and AFM-IR – have been applied to assess the role of chirality in the origins of circular dichroism in 2D nanoscale materials.
Chiral molecules are a unique type of molecules that possess a non-super imposable mirror image. These mirror images of chiral molecules are usually known as right handed and left handed, and because of the vector nature of light, they can exist with two forms of handedness – right and left circularly polarized.
Fully two-dimensional (2D) metamaterials, also called metasurfaces, contains planar-chiral plasmonic metamolecules that are just nanometers thick and have been shown to have chiral dichroism in transmission (CDT). According to theoretical calculations, this unexpected effect depends on finite non-radiative (Ohmic) losses of the metasurface. Considering the challenge of measuring non-radiative loss on the nanoscale, this surprising theoretical prediction has never been experimentally validated until now.
The s-SNOM technique is first used to map the optical energy distribution when the structures are subjected to RCP and LCP IR radiation, and then the AFM-IR technique is used to detect the markedly different Ohmic heating seen under LCP and RCP radiation.1 For the first time, it has been conclusively established that the circular dichroism seen in 2D metasurfaces is the result of handedness dependent Ohmic heating (Figure 6).
Analysis of Carbon Nanotubes with nanoIR
The AFM-IR method works by detecting the material’s thermal expansion caused by the absorption of infrared illumination.
Figure 6. Experimentally measured AFM cantilever deflection amplitudes. The cantilever deflection is directly proportional to temperature increase in the sample during the laser pulse; this confirms that the magnitude and spatial distribution of the Ohmic heating of a chiral 2D metasurface markedly depends on the handedness of light.
The thermal expansion of a material depends on a number of factors, such as the thickness of the material and the coefficient of thermal expansion. Single walled carbon nanotubes (CNT) and single layer graphene are examples of 1D and 2D materials that possess a low coefficient of thermal expansion and also have a thickness of about 1-2 nm. The nature of these 1D and 2D samples make characterization with AFM-IR challenging.
By placing a thin layer of polymeric material underneath graphene and CNT samples an increase of two orders of magnitude is seen in the AFM-IR signal intensity32.
As the thin sample absorbs the incident IR radiation, the generated heat is transferred to the thin polymer, which is observed to have a much higher coefficient of thermal expansion, and it expands. Shown in Figure 7 is the finite element analysis model used to replicate the effects of polymer thickness on both temperature changes and thermal expansion.
Figure 7. (a) Temperature rise (ΔT) and expansion (ΔZ) as a function of polymer thickness beneath the sample. (b) and (c) Temperature rise (b) with no polymer and (c) with polymer beneath the sample. (d) and (e) Vertical thermomechanical expansion (d) with no polymer and (e) with polymer beneath the sample.
Figure 8. (a) AFM topography imaging of CNTs deposited on polystyrene substrate. (b) IR chemical mapping image at 4000 cm- 1 showing absorption by CNTs. (c) IR chemical mapping image of monolayer graphene captured at 4000 cm-1.
This model was validated by checking an array of CNTs deposited on top of a 150 nm thick polystyrene layer on a ZnSe prism. Before CNT deposition, an area of the polymer substrate was removed to ensure that there was a CNT region without the polymer underneath.
As shown in Figure 8, the IR chemical image obtained at 4000 cm- 1 reveals clear signal from the CNT in the region that is supported by polystyrene, whereas no signal is seen where the polymer substrate has been removed. It has been proposed that the varying AFM-IR signal from different CNTs is caused by the difference between metallic and semiconducting tubes.
Shown in Figure 8c is the AFM-IR imaging of graphene on top of a 106 nm thick PMMA layer, demonstrating the extension of this method to monolayer 2D materials. When the AFM-IR signal is amplified by a thin layer of polymer, the signal intensity is shown to increase by two orders of magnitude.
This new technique allows for the AFM-IR characterization of 1 nm thick 2D and 1D materials that was previously impossible. Going forward, this remarkable signal improvement can be applied to a wide range of applications including ultrathin biologicals and a variety of 1D and 2D materials.
Investigating Exothermic Peaks of Polyethylene Using nanoTA and LCR
One of the most extensively utilized polymers is polyethylene (PE), which is used in many industries including 2D materials applications. In order to modify the electrical, thermal and mechanical properties of PE, inorganic fillers such as metallic particles and graphite have been added. Recently, hBN has shown huge potential as a filler because of its high thermal conductivity, mechanical strength and insulating properties.
At Sichuan University, Researchers used Lorentz Contact Resonance (LCR) and nano thermal analysis (nanoTA) to characterize this effect of hBN particles on the melting behavior of polyethylene3.
As illustrated in Figure 9a and b, LCR imaging clearly reveals the regions of high hBN concentration on the surface. Then using nanoTA, the softening temperature of various regions of the material was measured; when compared to areas without hBN, there was an increase in the transition temperature of 4-8 °C for the PE sample areas near hBN aggregates (Figure 9). As shown in Figure 9d, the accuracy of this method was verified when compared to conventional DSC analysis, with the bulk transition temperature being within the standard deviation of the nanoTA values.
Figure 9. (a) AFM mechanical image (using LCR) of the PE/BN composites, showing boron nitride clusters in the areas A,D and E; (b) LCR-AFM height image; (c) Local thermal analysis data of the assigned positions were obtained by nano-TA, comparing the melting temperatures of PE and BN; (d) DSC from the PE/BN composites (heating rate of 2 °C min-1).
These results, together with DSC analysis, demonstrate that during crystallization the meso-phase of the PE forms near the h-BN particles, inducing a weak exothermic peak that was not explained before. Figure 9 shows how nanoTA measurement was also conducted directly on the hBN particles, for which no thermal transition not measured at temperatures up to 400 °C.
Analyzing Thermal Conductivity of Graphene Sheets with SThM
Graphene, with its high thermal conductivity and potential in optoelectronics, has been a focus of recent research. Thermal conductivity of 2D materials is characterized by Scanning Thermal Microscopy (SThM), as it yields high sensitivity in resistance detection between the sample and the probe. These high spatial resolutions remove ambiguity in the detection of the source of a sample’s electrical capabilities. As a result, the SThM is a reliable method for monitoring a sample’s temperature and also thermal conductivity in a qualitative way.
At Lancaster University and Durham University, Researchers employed SThM to study the thermal conductivity on multilayer and single graphene sheets4. Graphene was first deposited on Si/SiO2 substrate with pre-patterned trenches, and with both graphene suspended over the trench and supported by the substrate imaged. It was observed that when the number of supported graphene layers was increased, it led to an obvious decrease in thermal resistance.
Interestingly, the thermal conductance of both multilayer and bilayer graphene suspended over the trench was observed to be greater than that of the supported layer, in contrast to expectations that conduction from the graphene to the substrate would generate greater heat dissipation.
Figure 10. (a) SThM image of supported graphene, showing varied thicknesses throughout the sample. (b) Measured contact thermal resistance as a function of the number of graphene layers, showing reduction in thermal resistance as the number of layers increases.
Since the mean free path of thermal phonons in graphene is relatively greater than the trench height, it is theorized that the major source of heat transfer were the ballistic acoustic phonons from the SThM tip, with 90% reaching the trench in the ballistic regime. Similar properties were exhibited by a graphene bulge still suspended over the trench, which ruled out experimental variations such as SThM contact area as the reason for such behavior.
These measurements show that when compared to single layer, three layer graphene had about 68% of the thermal conductance. Thermal mapping of border regions between the supported graphene layers also showed that the thermal transition region has a width of 50 to 100 nm, confirming theoretical estimates for the mean free path.
The nanoIR2-s system allows unique characterization of 2D material properties with complimentary near field s-SNOM and photothermal-based Tapping AFM-IR techniques.
AFM-based nanoscale property mapping offers correlative microscopy capability for thermal, electrical and mechanical property mapping.
1. Khanikaev AB, Arju N, Fan Z, Purtseladze D, Lu F, Lee J, Sarriugarte P, Schnell M, Hillenbrand R, Belkin MA, Shvets G. Experimental demonstration of the microscopic origin of circular dichroism in two-dimensional metamaterials. Nature Communications. 2017.
2. Rosenberger MR, Wang MC, Xie X, Rogers JA, N S, K WP. Measuring individual carbon nanotubes and single graphene sheets using atomic force microscope infrared spectroscopy. Nanotechnology. 2017.
3. Zhang X, Wu H, Guo S, Wang Y. 2015. Understanding in crystallization of polyethylene: the role of boron nitride (BN) particles. Royal Social of Chemistry Advances. 2015(121):99585-100407.
4. Pumarol ME, Rosamond MC, Tovee P, Petty MC, Zeze DA, Falko V, Kolosov OV. Direct Nanoscale Imaging of Ballistic and Diffusive Thermal Transport in Graphene Nanostructures. Nano Letters. 2012(12)2906-2911.
This information has been sourced, reviewed and adapted from materials provided by Anasys Instruments.
For more information on this source, please visit Anasys Instruments.